Development of the mammalian liver requires the rapid proliferation of hepatoblasts that is an integral feature of hepatogenesis. This proliferation is dependent upon the action of specific mitogens, such as hepatocyte growth factor, and mutations that affect hepatic mitogenic signaling pathways result in hypoplasia of the fetal liver. Not surprisingly, these factors are also important regulators of liver regeneration and have been implicated in progression of hepatocellular carcinomas. For these reasons the action of hepatic mitogens, and the signal transduction pathways they activate, have been studied intensely. One consequence of mitogenic stimulation is the onset of expression of immediate and delayed response genes that have important roles in controlling cell cycle progression. Most of these factors are expressed in all cell types and, therefore, do not specifically regulate hepatocyte proliferation. However, disruption of the pescadillo gene in zebrafish dramatically arrests fetal hepatoblast proliferation without affecting growth of the entire embryo. This implies that specific mechanisms exist to control proliferation of fetal liver cells during embryonic development. We have identified the mouse homolog of pescadillo, Pes1, and shown that it encodes a nuclear protein containing a BRCT domain. BRCT domains define a superfamily of proteins, including the breast cancer susceptibility protein BRCA1, which have diverse roles in cell proliferation. Based on our preliminary studies and the phenotype of pescadillo-deficient zebrafish embryos, we hypothesize that Pes1 protein specifically regulates proliferation of hepatic cells during development by controlling the action or expression of proteins involved in cell cycle progression. We, therefore, propose to test this hypothesis by i) determining the physical and biochemical properties of Pes1 protein that are important for its function, ii) elucidating the molecular pathways that utilize Pes1 function by identifying Pes1 binding partners, and iii) examining the physiological role of Pes1 during embryogenesis using Pes1 "knockout" mice.